MicroBlaze Processor Reference Guide
57
UG081 (v14.7)
Virtual-Memory Management
System software can use the PID to uniquely identify software processes (tasks, subroutines,
threads) running on the processor. Independently compiled processes can operate in effective-
address regions that overlap each other. This overlap must be resolved by system software if
multitasking is supported. Assigning a PID to each process enables system software to resolve the
overlap by relocating each process into a unique region of virtual-address space. The virtual-address
space mappings enable independent translation of each process into the physical-address space.
Page-Translation Table
The page-translation table is a software-defined and software-managed data structure containing
page translations. The requirement for software-managed page translation represents an
architectural trade-off targeted at embedded-system applications. Embedded systems tend to have a
tightly controlled operating environment and a well-defined set of application software. That
environment enables virtual-memory management to be optimized for each embedded system in the
following ways:
•
The page-translation table can be organized to maximize page-table search performance (also
called table walking) so that a given page-translation entry is located quickly. Most general-
purpose processors implement either an indexed page table (simple search method, large page-
table size) or a hashed page table (complex search method, small page-table size). With
software table walking, any hybrid organization can be employed that suits the particular
embedded system. Both the page-table size and access time can be optimized.
•
Independent page sizes can be used for application modules, device drivers, system service
routines, and data. Independent page-size selection enables system software to more efficiently
use memory by reducing fragmentation (unused memory). For example, a large data structure
can be allocated to a 16 MB page and a small I/O device-driver can be allocated to a 1 KB
page.
•
Page replacement can be tuned to minimize the occurrence of missing page translations. As
described in the following section, the most-frequently used page translations are stored in the
translation look-aside buffer (TLB). Software is responsible for deciding which translations are
stored in the TLB and which translations are replaced when a new translation is required. The
replacement strategy can be tuned to avoid thrashing, whereby page-translation entries are
constantly being moved in and out of the TLB. The replacement strategy can also be tuned to
prevent replacement of critical-page translations, a process sometimes referred to as page
locking.
The unified 64-entry TLB, managed by software, caches a subset of instruction and data page-
translation entries accessible by the MMU. Software is responsible for reading entries from the
page-translation table in system memory and storing them in the TLB. The following section
describes the unified TLB in more detail. Internally, the MMU also contains shadow TLBs for
instructions and data, with sizes configurable by
C_MMU_ITLB_SIZE
and
C_MMU_DTLB_SIZE
respectively.
These shadow TLBs are managed entirely by the processor (transparent to software) and are used to
minimize access conflicts with the unified TLB.
Translation Look-Aside Buffer
The translation look-aside buffer (TLB) is used by the MicroBlaze MMU for address translation
when the processor is running in virtual mode, memory protection, and storage control. Each entry
within the TLB contains the information necessary to identify a virtual page (PID and effective page
number), specify its translation into a physical page, determine the protection characteristics of the
page, and specify the storage attributes associated with the page.
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